As a conventional image display apparatus, a liquid crystal display apparatus of an active matrix driving method is known. This liquid crystal display apparatus is, as shown in FIG. 23, provided with a pixel array (ARY) 101, a scanning signal line drive circuit (GD) 102, a data signal line drive circuit (SD) 103, a timing signal generating circuit (CTL) 104, and a video signal processing circuit (SIG) 105.
The pixel array 101 is provided with a large number of scanning signal lines GL and a large number of data signal lines SL that are crossed with each other, and each intersection of each scanning signal line GL and each data signal line SL is accordingly provided for a pixel (PIX) 106. Namely, each area surrounded with the adjacent two scanning signal lines GL and the adjacent two data signal lines SL is provided with each pixel 106, and the pixels 106 that are arranged in a matrix manner compose a display screen.
The scanning signal line drive circuit 102, synchronizing to a timing signal such as a clock signal GCK inputted from the timing signal generating circuit 104, sequentially selects the scanning signal lines GL and controls opening and closing of a switching element in the pixels 106. By doing this, the scanning signal line drive circuit 102 writes into each pixel 106, video signals (data) that are written into each data signal line SL, and keeps the data that are written into each pixel 106.
The data signal line drive circuit 103, synchronizing to a timing signal such as a clock signal SCK inputted from the timing signal generating circuit 104, samples a video signal DAT inputted from the video signal processing circuit 105, and amplifies the video signal DAT, if necessary, and writes the video signal DAT into each data signal line SL.
As shown in FIG. 24, each pixel 106 in FIG. 23 is composed of a field-effect transistor SW, which is a switching element, and a pixel capacitor (which includes a liquid crystal capacitor CL, and a supplemental capacitor CST, that is added if necessary). In FIG. 24, one electrode of the pixel capacitor is connected to the data signal line SL via a drain and a source of the transistor SW. A gate of the transistor SW is connected to the scanning signal line GL. The other electrode of the pixel capacitor is connected to a common electrode line that is common to all pixels. Then voltage that is applied to each liquid crystal capacitor CL modulates transmittance or reflectance of the liquid crystal, and display is performed by using the modulated transmittance or reflectance.
In addition, a technology has been recently developed to integrate the pixel array 101 and the drive circuits 102 and 103 on the same substrate, for achieving a liquid crystal display apparatus of a smaller size and higher resolution and a lower mounting cost.
In the liquid crystal display apparatus of the drive circuit integrated type like this, when realizing a liquid crystal display apparatus of a transparent type, which is now widely used, its substrate needs to be a quartz substrate or a glass substrate, which is a transparent substrate. Further, in case the circuit is formed on the quartz substrate or the glass substrate, a polycrystalline silicone thin film transistor, which can be manufactured at a manufacturing temperature of no more than 600° C., is used as an active element, in view of heat resistance of the substrate.
FIG. 25 is a diagram illustrating an example of the liquid crystal display apparatus of the drive circuit integrated type. In the liquid crystal display apparatus, the pixel array 101, the scanning signal line drive circuit 102, and the data signal line drive circuit 103 are formed on a substrate (SUB) 107. In addition, provided on the substrate 107 is a precharge circuit (PC) 108, which is provided if the data signal line drive circuit 103, which is composed of the polycrystalline silicone thin film transistor, has low driving ability and its writing of data into the data signal lines SL thus needs assistance.
Next, driving methods of the data signal lines are explained. Analog driving methods include an analog point-by-point driving method and an analog line-by-line driving method, whereas digital driving methods include a selector type driving method, an R-DAC type driving method, and a C-DAC type driving method.
Among these driving methods, the analog line-by-line driving method, the selector type driving method, the R-DAC type driving method, and the C-DAC type driving method have following difficulties, when applied to the liquid crystal display apparatus of the drive circuit integrated type; it is difficult to locate on the substrate due to their strict design rules, it is difficult to respond to multiple tone gradation display, or they cause degradation of their display qualities.
In other words, in the liquid crystal display apparatus of the drive circuit integrated type, the polycrystalline silicone thin film is used for a semiconductor layer in the circuit as described above, but occupies a larger location area on the substrate in comparison to a mono-crystalline silicone.
Furthermore, more specifically, in the analog line-by-line driving method, an amplifier of high precision is required for amplifying the inputted video signals, but it is difficult to form the amplifier of high precision in a small area by using a polycrystalline silicone as a material of the semiconductor.
Besides, in the R-DAC type driving method and the C-DAC type driving method, reference voltage for displaying multiple tone gradations is generated by voltage dividing by dividing resistance or capacity. However, when an element of the resistance or the capacity, which is used as these voltage dividing means, is made of the polycrystalline silicone thin film, it is difficult to form the element in a small area. In addition, since the resistance or the capacity made of the polycrystalline silicone thin film has large property unevenness, it is impossible to achieve voltage dividing ratio as designed, thus degrading the display quality. Note that, when the elements using the polycrystalline silicone as the material of the semiconductor compose the drive circuit, the drive circuit needs to be composed of only logic elements, to prevent degradation of the display quality due to the property unevenness among the elements.
Moreover, in the selector type driving method, the reference voltage inputted from outside is supplied to the data signal lines SL via a selector circuit, corresponding to the video signals, and its circuit includes only a logic circuit and a transfer switch. Therefore, the selector type driving method has the simplest circuit structure among the digital driving methods. On the other hand, since a reference voltage source is required outside to supply an enough reference voltage to respond to the display tone gradations, it is only possible in practical use to obtain eight through sixteen tone gradations. This becomes a significant disadvantage in case a large number of tone gradations are displayed.
Because of the above reasons, for performing display having a further more number of tone gradations, on the liquid crystal display apparatus of the drive circuit integrated type, the analog line-by-line driving method, the selector type driving method, the R-DAC type driving method, or the C-DAC type driving method are not employed, but the analog point-by-point driving method is most generally used.
Here, the data signal line drive circuit in the analog point-by-point driving method is explained. In the data signal line drive circuit of the analog point-by-point driving method, as shown in FIG. 26, the inputted video signals DAT are written into the data signal lines SL, by opening and closing sampling circuits AS synchronously to an output pulse of each stage of flipflops (FFs) that constitutes a shift register.
More specifically, because the data signal line drive circuit of the analog point-by-point driving method only carries out transfer of the video signal DAT inputted from the outside to the data signal lines, its circuit structure is very simple so that the data signal line drive circuit of this kind can be used in the liquid crystal display apparatus of the drive circuit integrated type, while displaying multiple tone gradations without degrading the display quality.
In the data signal line drive circuit of the analog point-by-point driving method, however, an analog video signal output circuit with high driving ability is externally, thereby causing problems of increasing its electric power consumption as a system and significantly increasing its cost.
Furthermore, the drive circuit of the above described analog point-by-point driving method is provided with no digital interface. For this reason, even the liquid crystal display apparatus driven with an input of a digital signal requires a D/A (digital/analog) converting circuit externally to a display panel, in which the pixel array and the drive circuit are formed on the same substrate, thus further increasing the cost.
Here, as a driving method that includes the digital interface with low electric power consumption and an ability of displaying the multiple tone gradations in high display quality even when the polycrystalline silicone is used as the material of the semiconductor, there is a driving method using pseudo tone gradation processing.
Here an example of an arrangement of the conventional drive circuit using the pseudo tone gradation processing is illustrated in FIG. 27. In the data signal line drive circuit using the pseudo tone gradation processing, as shown in FIG. 27, the inputted digital video signals DAT are latched into a latch LAT, synchronizing to the output pulse of each stage of flipflops (FFs) that constitutes a shift register. The latched video signals are decoded by a decoder circuit DEC and the decoded video signals are subjected to the pseudo tone gradation processing in a line-by-line manner.
Here, the pseudo tone gradation processing in the arrangement of FIG. 27 is briefly explained as follows. The present pseudo tone gradation processing, by rounding off a less significant bit after a fixed noise pattern is superimposed on an image data, enables a low-bit drive circuit to display an image having more bits in a pseudo manner. The present pseudo tone gradation processing is one of the simplest arrangement among the pseudo tone gradation processing. In an image display apparatus of high definition, since the method to increase a number of tone gradations in the pseudo manner does not significantly degrade the image quality, its effect causes no problem in many cases.
In the arrangement of FIG. 27, the inputted video signals DAT and the fixed noise pattern memorized in a memory ROM are added together by an ADDER for each video signal to be respectively outputted to each data signal line, then subjected to an exception processing in case such as an overflow by an exception processing circuit OFP, and its less significant bit is rounded off by a quantization circuit QNT. According to the video signals subjected to the pseudo tone gradation processing like this, one of the reference voltages VREF corresponding to the video signals is selectively supplied to the data signal line SL by a selector circuit SEL.
As described above, the drive circuit using the pseudo tone gradation processing is provided with the digital interface, and can display multiple tone gradations in high display quality even when the polycrystalline silicone is used as the material of the semiconductor, and consumes a relatively small amount of electric power.
Nonetheless, since arrangements relating to the pseudo tone gradation processing, which are the adder ADDER, the exception processing circuit OFP, and the quantization circuit QNT, are provided to each data signal line. Therefore, the display apparatus of the drive circuit integrated type in which the pixel array and the drive circuit are formed on the same substrate should have a drive circuit having a very complicated arrangement. For this reason, when the drive circuit is composed of the element using the polycrystalline silicone as the material of the semiconductor, the drive circuit becomes too large, which causes problems such that manufacture of the drive circuit is practically difficult.